The present invention deals with DC to DC time ratio or "chopper" control systems and, more particularly, with means for effecting the operation of a switch bypassing the repetitively-actuated switching means of the time ratio controller.
Control of the operation of modern-day battery-powered vehicles is commonly achieved through the use of time ratio controllers, sometimes referred to as chopper systems. In such a system, electronic switching means are interposed between the traction motor and a source of DC voltage. The switching means are operated at an extremely high rate, the ratio of conductive to non-conductive time (also known as the mark-space ratio) determining the percent of available voltage to be applied across the motor. Early in the development of time ratio controllers it was recognized that, due to the resistive drop therethrough and the heating effected thereby, the use of such electronic switching means is inefficient as the mark-space ratios approach 100%. For this reason, it has been found practical to provide a mechanical switch, termed a bypass contactor, in shunt about the electronic switching means.
Typically the referred-to-arrangement takes the form of a thyristor such as an SCR which is operated by a variable oscillator for affording pulse control of a traction motor. A pair of electro-magnetically-operated contacts in the form of a standard circuit contactor are connected across the SCR and the winding for closing the contacts is coupled in a circuit with a switch which is operated in response to certain circuit parameters.
In one straightforward application the bypass contactor is operated simply by coupling a "kickdown switch," or bypass demand switch, to the vehicle speed control mechanism. In this manner, when the speed control is adjusted for maximum power the bypass contactor is automatically actuated. However, should the vehicle be operating at low speed and the bypass contactor closed before full torque has been developed by the motor, a severe jerk in operation of the vehicle will occur. Aside from being detrimental to the drive train components, a sudden lurch of the vehicle could be dangerous to the driver or to bystanders. To overcome the foregoing problem, it has become customary to insert a time delay mechanism between the demand switch and the contactor actuation means. It is then assumed that after the time delay times out the vehicle will be in a proper state for accepting closure of the bypass contactor, and thus full source voltage, across the traction motor. Other difficulties inhere in this approach, however. For instance, the bypass contactor cannot be closed until the speed control is at its utmost position, regardless of other motor control system parameters. Further, the contactor may be inadvertently closed before the field winding directional contactors are operated, causing the vehicle to suddently lurch ahead at full power upon energization of the field into the circuit. While various compensating systems have been built, they commonly have had the disadvantage of not allowing the bypass contactor to close until a substantial amount of current is already flowing in the system. In instances where the main SCR's become overheated, it is common to cut back the amount of allowable current. This tends to accentuate the heating problem, however, since the lessened current flow may make it impossible to close the bypass contactor and thus provide a cooling respite for the SCR.
It will therefore be seen that it would be desirable to provide a system which effects the closure of a bypass contactor under all practicable conditions, while normally assuring a smooth transition in torque applied to the drive system. It is therefore an object of the present invention to provide improved means for effecting the operation of a bypass contactor in a traction motor pulse control system.
It is another object of the present invention to provide means for closing a bypass contactor, which assures a smooth transition in generated torque.